1. The effect of diets, varying in quantity or quality of protein, on white blood cell regeneration was studied in leukopenic rats, the leukopenia having been induced by a protein-free diet.

2. Diets containing different amounts of casein (3, 6, 9 and 18 per cent, respectively), were fed ad libitum. At the 3 per cent level, a further decrease occurred of white blood cells, whereas the other three diets initiated a regeneration of leukocytes, its degree being more or less in proportion to the casein content.

3. In experiments with diets containing 18 and 30 per cent of casein, the amount of protein eaten and not its level in diet was the decisive factor in the regeneration of leukocytes. The white blood cell regenerating effect of a diet containing an optimal level of protein, may be neutralized when given in restricted amounts.

4. Diets containing nutritionally inferior proteins, fed at 9 per cent level, also impaired normal regeneration of leukocytes. The white blood cell regeneration afforded by the proteins investigated was found to increase in the following order: maize, gelatin, wheat, casein, processed soya, peanut, meat, egg.

5. In white blood cell regeneration promoted by dietary protein, granulocytes were found to react to a greater degree than lymphocytes and monocytes.

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